Résumé
Leucogranites plutons related to the Hercynian orogeny crop out over large areas in the French Massif Central hiding the early structuration phases. Their emplacement is related to Carboniferous post-collisional thinning coeval with a ductile deformation characterized by a NW-SE stretching. The Millevaches Massif (MV) is located in the northern part of the Massif Central. Its N-S elongated shape strikes perpendicular to the usual E-W trend of the Hercynian belt. The MV consists of several porphyritic biotite granites and leucogranites plutons hosted in micaschists forming, N-S or NW-SE elongated stripes. Field relationships show that the biotite granite is older than the leucogranites. This huge granite body is affected by large ductile shear zones on its edges and in its inner part. These shear zones played a significant role in magma emplacement and exhumation of the Massif. In order to get a detailed 3D image of the buried structures, we carried out a gravity survey on the NE part of the MV together with a regional density measurements. First modelling suggest that the MV is a laccolite from 2 up to 4 km thick ( from N to S), rooting down to about 6 km depth, at east and south edges. New AMS measurements (controlled by micas orientation) in the north-central part of the MV complete available data to the north. The magnetic foliation pattern shows a general sub horizontal dip, increasing near the edges. Lineations are sub-horizontal. The magnetic lineation pattern reveals three main directions: i) N-S, more often observed in the porphyritic biotite granites, ii) NW-SE mainly found in the leucogranites, iii) NE-SW restricted to the leucogranite. The NE-SW direction could be related to a late (i.e.: post solidus) ductile deformation phase with a low angle top-to-the-SW shearing, mainly detected in the leucogranites to the NE. In large scale, the NW-SE trend complies with the stretching direction of the Middle Carboniferous late-orogenic extension. The N-S trend is not yet understood. If the observed horizontal lineation is of magmatic origin it would suggest that the magma was emplaced as a laccolite into the foliation planes. On the other hand, new 39Ar/40Ar results obtained on the large shear zones are essential to understand the origin and the geodynamic context of MV emplacement. Finally and as a working hypothesis, we propose to interpret the MV as a huge laccolitic intrusion driven and emplaced above pre-existent N-S trending vertical lineament.